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Seasonal prediction of the East Asian summer monsoon (EASM) strength is probably one of the most challenging and crucial issues for climate prediction over East Asia. In this paper, a statistical method called partial-least square (PLS) regression is utilized to uncover principal sea surface temperature (SST) modes in the winter preceding the EASM. Results show that the SST pattern of the first PLS mode is associated with the turnabout of warming (or cooling) phase of a mega-El Niño/Southern Oscillation (mega-ENSO) (a leading mode of interannual-to-interdecadal variations of global SST), whereas that of the second PLS mode leads the warming/cooling mega-ENSO by about 1 year, signaling precursory conditions for mega-ENSO. These indicate that mega-ENSO may provide a critical predictability source for the EASM strength. Based on a 40-year training period (1958-1997), a PLS prediction model is constructed using the two leading PLS modes and 3-month-lead hindcasts are performed for the validation period of 1998-2013. A promising skill is obtained, which is comparable to the ensemble mean of versions 3 and 4 of the Canadian Community Atmosphere Model (CanCM3/4) hindcasts from the newly developed North American Multi-model Ensemble Prediction System regarding the interannual variations of the EASM strength. How to improve dynamical model simulation of the EASM is also examined through comparing the CanCM3/4 hindcast (1982-2010) with the 106-year historical run (1900-2005) by the Second Generation Canadian Earth System Model (CanESM2). CanCM3/4 exhibits a high skill in the EASM hindcast period 1982-2010 during which it also has a better performance in capturing the relationship between the EASM and mega-ENSO. By contrast, the simulation skill of CanESM2 is quite low and it is unable to reproduce the linkage between the EASM and mega-ENSO. All these results emphasize importance of mega-ENSO in seasonal prediction and dynamical model simulation of the EASM. [ABSTRACT FROM AUTHOR]

A large intraseasonal variation in geopotential height over the Central Asia region, where the Asian subtropical jet is located, occurs between May and June, and the most dominant variation has a wave-like distribution. This variation in geopotential height influences precipitation across South and Southeast Asia. In this paper, we use composite analysis to determine the causes of this intraseasonal variation over Central Asia. The wave train propagates from the northern Atlantic Ocean to Central Asia over a period of a week, and generates an anomaly in geopotential height over the region. The tropical disturbance, which is similar to the Madden-Julian oscillation, appears a few days before the maximum of the anticyclonic anomaly over Central Asia, and is accompanied by active convection over the Indian Ocean and suppressed convection over Central America. Results of numerical experiments using a linear baroclinic model show that the active convection over the northern Indian Ocean causes the anticyclonic anomaly over Central Asia. The wave train that extends from the northern Atlantic Ocean to Central Asia is generated by negative thermal forcing over Central America, and the phase distribution of this wave train is similar to that observed in the composite analysis. Central Asia is the region where the effects of the tropics and middle latitudes overlap, and it is an important connection point between the Asian monsoon and middle latitudes. [ABSTRACT FROM AUTHOR]

Surface air temperature is an important factor for human quality of life and is a key marker of global climate change. Understanding multidecadal changes in surface air temperature, and accurately predicting future trends, are therefore important for economic development. In this work, we explore multidecadal variability in East Asian surface air temperature (EASAT). We find that EASAT shows a strong multidecadal variability between 1900 and 2017. Observational analysis shows that annual EASAT multidecadal variability is highly associated with the North Atlantic oscillation (NAO) and the NAO leads detrended annual EASAT by 15–20 years. Further analysis illustrates that the NAO precedes annual EASAT multidecadal variability through its leading effect on the Atlantic Multidecadal oscillation (AMO). The AMO influences annual EASAT multidecadal variability through the Africa–Asia multidecadal teleconnection (AAMT) pattern. An NAO-based linear model is therefore established to predict annual EASAT. The model is able to better hindcast annual EASAT based on different periods of the time-series. Due to the joint influences of NAO multidecadal variability and the forcing associated with anthropogenic greenhouse gas emissions, annual EASAT for 2018–2034 is predicted to remain at its current level or even slightly lower, followed by a period of fast warming over the following decades. [ABSTRACT FROM AUTHOR]

It is well recognized that El Niño-Southern Oscillation (ENSO) may exert a direct impact on the East Asian summer monsoon rainfall through modulating the Philippine Sea anticyclone variability. Such ENSO associated influence is evident in the monsoon region, i.e., Southeast China, the Yangtze River, Korean Peninsula and Japan. It remains unclear whether and how this ENSO related effect can reach the Yellow River region, a monsoon/arid transition region. In this study, results show that the year-to-year variations of the Yellow River summer rainfall can be indirectly influenced by ENSO, during its developing phase. The western Tibetan Plateau snow cover (WTPSC) may act as a “capacitor”, helping ENSO signal to reach the Yellow River region. During the El Niño developing spring, the associated diabatic heating in Pacific region can excite an anomalous cyclone over the plateau and anomalous upward flows over the western plateau. Such circulation configuration favors an excessive WTPSC anomaly in spring. The more WTPSC may increase the surface albedo, decrease the absorbed net shortwave radiation and in turn intensify the WTPSC. Through such snow-albedo feedback process, the excessive WTPSC anomaly may strengthen and persist through summer, which may induce two noticeable wave trains in the upper and lower troposphere propagating northeastward to the Yellow River region. Associated with the wave trains, a low pressure anomaly prevails over northeast China. To the southwest side of the anomalous low pressure, the abnormal northerly wind may bring large volumes of dry cold air with little moisture to the Yellow River region, leading to the anomalous drought there. During the La Niña developing summer, the situation tends to be opposite. As such, the ENSO associated influence is tied to the interannual variations of the following summer Yellow River precipitation, with the development of ENSO from spring. [ABSTRACT FROM AUTHOR]

The subseasonal migrations of the East Asian summer monsoon are nearly identical to that of the South Asian summer monsoon. In mid-May to mid-June (Phase 1), the South Asian westerly strengthens, the center of the South Asian high moves northwestward, and the East Asian frontal system coupled to and located north of the Western North Pacific (WNP) high moves northward, with all of these actions occurring rapidly. In mid-June to late July (Phase 2), the strength of the South Asian westerly reaches a maximum, the center of the South Asian high remains at approximately 30°N, and the northward propagation of the WNP high becomes stagnant. The speed of the northward movement of the WNP high in Phase 2 is only half the speed of that in Phase 1. By late June, the South Asian monsoon has reached northern India and been blocked by the Himalayas. This indicates that the Himalayas have an effect of constraining the speed of the northward movement of the South Asian high and, in turn, the WNP high. Climate model experiments further reveal that the near-stationary nature of the East Asian frontal system in Phase 2 is related to the blocking of the South Asian summer monsoon by the Himalayas. We suggest that the evolution of the WNP high is constrained by the evolution of the South Asian high, and the Meiyu–Baiu is connected to the South Asian summer monsoon through the impact of the South Asian monsoon heating on the upper tropospheric circulation. [ABSTRACT FROM AUTHOR]

This study investigates the concurrent variations in the location and intensity of the East Asian subtropical jet (EASJ) and polar front jet (EAPJ) with a focus on the relationship between the variations and the atmospheric circulation in the mid-latitude region. The possible mechanisms for the concurrent variations of the EASJ and EAPJ are explored. The wintertime upper-level zonal wind variation over the landmass (50°-100°E) is characterized by two principal modes, i.e., the out-of-phase variations in the intensity and the meridional displacements of the EASJ/EAPJ. The EASJ and EAPJ are intensified (weakened) when they are located close to (far away from) each other, corresponding to a weakened (intensified) East Asian trough over the northern Pacific region and a decreased (increased) geopotential height in the mid-latitude over central Asia. The spatial pattern of 500 hPa geopotential height anomaly is similar to the Eurasian teleconnection pattern, while the pattern of sea level pressure anomaly shows a feature similar to the Arctic oscillation. Analyses of related thermal-dynamical processes indicate that the increased (decreased) diabatic heating and horizontal heat transport lead to a larger (smaller) meridional temperature gradient over the EASJ/EAPJ regions, which subsequently intensifies (weakens) the East Asian jets. Severe (weak) convective activities accompanied with warmer (colder) SST in the tropics also contribute to the intensification (weakening) of the EASJ and EAPJ. Dynamically, the strong (weak) divergence of the eddy vorticity flux along the northern Tibetan Plateau and that of the Eliassen-Palm flux of the synoptic-scale stationary waves are favorable for the intensification (weakening) of zonal winds through the wave-mean flow interactions. As a result, the EASJ and EAPJ are intensified (weakened). [ABSTRACT FROM AUTHOR]

The response of monsoon precipitation to global warming, which is one of the most significant climate change signals at the earth's surface, exhibits very distinct regional features, especially over the South China Sea (SCS) and adjacent regions in boreal summer. To understand the possible atmospheric dynamics in these specific regions under the global warming background, changes in atmospheric heating and their possible influences on Asian summer climate are investigated by both observational diagnosis and numerical simulations. Results indicate that heating in the middle troposphere has intensified in the SCS and western Pacific regions in boreal summer, accompanied by increased precipitation, cloud cover, and lower-tropospheric convergence and decreased sea level pressure. Sensitivity experiments show that middle and upper tropospheric heating causes an east-west feedback pattern between SCS and western Pacific and continental South Asia, which strengthens the South Asian High in the upper troposphere and moist convergence in the lower troposphere, consequently forcing a descending motion and adiabatic warming over continental South Asia. When air-sea interaction is considered, the simulation results are overall more similar to observations, and in particular the bias of precipitation over the Indian Ocean simulated by AGCMs has been reduced. The result highlights the important role of air-sea interaction in understanding the changes in Asian climate. [ABSTRACT FROM AUTHOR]

A recently developed method of climatic relevant singular vectors (CSVs) is applied to an atmospheric general circulation model (CAM4) to investigate the optimal error growth of the South Asian monsoon (SAM) seasonal prediction due to uncertainties in initial conditions of the sea surface temperature (SST). Emphasis is placed on the investigation of the optimal error growth of SAM seasonal forecast due to the SST uncertainties in the Indian and the equatorial Pacific Oceans. It is found that the uncertainties in the Indian Ocean can result in much larger error growth of SAM seasonal prediction than those in equatorial Pacific Ocean. Most of the CSV patterns over the Indian Ocean resembled a dipole-like structure with opposite signs spanning the northern and southern Indian Ocean. It is seen that the CSVs error growth rate changes significantly depending on the initial states whereas the CSVs patterns are insensitive to the initial conditions. The CSV patterns and error growth rates, calculated using CAM4, are also compared against those using coupled model CCSM4, indicating that the CSVs patterns from CAM4 are similar to those from CCSM4 coupled model while the error growth rate is lower in CAM4 than in CCSM4. Ensemble summer hindcasts, for the period from 2000 to 2009 over Indian Ocean, are constructed by the CSVs. For the purpose of comparison, ensemble forecasts constructed by the time lag ensemble (TLE) method are also conducted. It is found that the ensemble mean prediction by CSVs has a better skill than both the prediction by TLE and by the control run, in particular for predictions longer than 3 months, indicating the merit of CSV for SAM ensemble forecast. [ABSTRACT FROM AUTHOR]

Long-term changes in surface wind speed (SWS) are influenced by both large-scale circulation and relative resistance. The effects of large-scale circulation are embodied by the pressure-gradient force (PGF), which is mostly a natural factor, whereas the resistance is due to the drag between the air and the surface as well as in the different boundary layers, which is mainly caused by the anthropogenic land use and cover change (LUCC). We performed experiments using a simple dynamical method in which a balance among the PGF, Coriolis force, and drag is reached to separate the effects of the PGF and LUCC on the SWS, and then, to quantitatively estimate the influence of the LUCC on the SWS over the East China Plain (ECP) during the period 1980-2011. The results show a distinct decrease in the SWS in the station observation data with a rate of −0.13 m s (10 year), but there is no statistically significant long-term trend in the reanalysis data. At the same time, the drag coefficient induced by the LUCC shows an increasing trend, which is consistent with the 30 % increase in the rate of urbanization during the study period. In addition, the PGF fluctuates with distinct seasonal and interannual changes, and it has an insignificant long-term increasing trend during the period 1980-2011. At the same time, the spatial distribution of the linear trend coefficient of the normalized PGF is inconsistent with that of the SWS, but the linear trend coefficient of the normalized drag coefficient shows a similar spatial distribution as the SWS. Therefore, the increase in the drag coefficient induced by the LUCC should account for the long-term decrease in the SWS. The difference between the model wind speed, in which the drag coefficient is constrained to its value in the year 1980, and the observed wind speed at each station (SWSD) can reflect the influence of the LUCC on the SWS. Furthermore, the long-term changes in East Asian monsoons may not completely account for the observed wind speed decrease near the surface in the ECP region, but it is an important factor in the SWS. [ABSTRACT FROM AUTHOR]

This study analyzes areal changes and regional climate variations in global semi-arid regions over 61 years (1948-2008) and investigates the dynamics of global semi-arid climate change. The results reveal that the largest expansion of drylands has occurred in semi-arid regions since the early 1960s. This expansion of semi-arid regions accounts for more than half of the total dryland expansion. The area of semi-arid regions in the most recent 15 years studied (1990-2004) is 7 % larger than that during the first 15 years (1948-1962) of the study period; this expansion totaled 0.4 × 10 and 1.2 × 10 km within the American continents and in the Eastern Hemisphere, respectively. Although semi-arid expansion occurred in both regions, the shifting patterns of the expansion are different. Across the American continents, the newly formed semi-arid regions developed from arid regions, in which the climate became wetter. Conversely, in the continental Eastern Hemisphere, semi-arid regions replaced sub-humid/humid regions, in which the climate became drier. The climate change in drying semi-arid regions over East Asia is primarily dominated by a weakened East Asian summer monsoon, while the wetting of semi-arid regions over North America is primarily controlled by enhanced westerlies. [ABSTRACT FROM AUTHOR]

Based on multiple sources of atmospheric and oceanic data, this study performs a series of composite analysis of the South Asian summer monsoon (SASM) onset against ENSO events, and indicates that warm/cold ENSO events induce later/earlier onset of the SASM by modulating the vertical coupling of the upper- and lower-level circulation over the South Asia. Specifically, during the monsoon onset of Bay of Bengal (BOB), the ENSO-induced convection anomalies over the southern Philippines can modulate the position of South Asian high (SAH) in late April in the upper troposphere, which evolves to affect the monsoon onset convection by changing the upper divergence-pumping effect. In the lower troposphere, ENSO induces an anomalous zonal gradient of sea surface temperature (SST) over the Indian-western Pacific Ocean to alter the barotropic instability which further affects the formation of BOB monsoon onset convection. During the Indian summer monsoon onset, the anomalous convection over northeastern BOB and Indochina Peninsula in late May act to change the SAH position and its relevant upper divergence-pumping over the Arabian Sea (AS). Meanwhile, the Indian monsoon onset convection is also modulated by the ENSO-induced changes in intensity of the inertial instability and the forced convection over the AS, which are related to an ENSO-induced anomalous cross-equatorial SST gradient and zonally asymmetric meridional gradient of sea level pressure, and an anomalous westerly over the central AS in the lower troposphere. Results demonstrate that during the BOB and India monsoon onset, the influences of ENSO on the upper circulation are similar, but are distinctly different on the lower-level circulation. [ABSTRACT FROM AUTHOR]

The northern fringe of the Asian summer monsoon region (NASM) in China refers to the most northwestern extent of the Asian summer monsoon. Understanding the characteristics and underlying mechanisms of drought variability at long and short time-scales in the NASM region is of great importance, because present and future water shortages are of great concern. Here, we used newly developed and existing tree-ring, historical documentary and instrumental data available for the region to identify spatial and temporal patterns, and possible mechanisms of drought variability, over the past two millennia. We found that drought variations were roughly consistent in the western (the Qilian Mountains and Hexi Corridor) and eastern (the Great Bend of the Yellow River, referred to as GBYR) parts of the NASM on decadal to centennial timescales. We also identified the spatial extent of typical multi-decadal GBYR drought events based on historical dryness/wetness data and the Monsoon Asia Drought Atlas. It was found that the two periods of drought, in AD 1625-1644 and 1975-1999, exhibited similar patterns: specifically, a wet west and a dry east in the NASM. Spatial characteristics of wetness and dryness were also broadly similar over these two periods, such that when drought occurred in the Karakoram Mountains, western Tianshan Mountains, the Pamirs, Mongolia, most of East Asia, the eastern Himalayas and Southeast Asia, a wet climate dominated in most parts of the Indian subcontinent. We suggest that the warm temperature anomalies in the tropical Pacific might have been mainly responsible for the recent 1975-1999 drought. Possible causes of the drought of 1625-1644 were the combined effects of the weakened Asian summer monsoon and an associated southward shift of the Pacific Intertropical Convergence Zone. These changes occurred due to a combination of Tibetan Plateau cooling together with more general Northern Hemisphere cooling, rather than being solely due to changes in the sea surface temperature of the tropical Pacific. Our results provide a benchmark for comparing and validating paleo-simulations from general circulation model of the variability of the Asian summer monsoon at decadal to centennial timescales. [ABSTRACT FROM AUTHOR]

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958-2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter. [ABSTRACT FROM AUTHOR]

The boreal summer Asian monsoon has been evaluated in 25 Coupled Model Intercomparison Project-5 (CMIP5) and 22 CMIP3 GCM simulations of the late twentieth Century. Diagnostics and skill metrics have been calculated to assess the time-mean, climatological annual cycle, interannual variability, and intraseasonal variability. Progress has been made in modeling these aspects of the monsoon, though there is no single model that best represents all of these aspects of the monsoon. The CMIP5 multi-model mean (MMM) is more skillful than the CMIP3 MMM for all diagnostics in terms of the skill of simulating pattern correlations with respect to observations. Additionally, for rainfall/convection the MMM outperforms the individual models for the time mean, the interannual variability of the East Asian monsoon, and intraseasonal variability. The pattern correlation of the time (pentad) of monsoon peak and withdrawal is better simulated than that of monsoon onset. The onset of the monsoon over India is typically too late in the models. The extension of the monsoon over eastern China, Korea, and Japan is underestimated, while it is overestimated over the subtropical western/central Pacific Ocean. The anti-correlation between anomalies of all-India rainfall and Niño3.4 sea surface temperature is overly strong in CMIP3 and typically too weak in CMIP5. For both the ENSO-monsoon teleconnection and the East Asian zonal wind-rainfall teleconnection, the MMM interannual rainfall anomalies are weak compared to observations. Though simulation of intraseasonal variability remains problematic, several models show improved skill at representing the northward propagation of convection and the development of the tilted band of convection that extends from India to the equatorial west Pacific. The MMM also well represents the space-time evolution of intraseasonal outgoing longwave radiation anomalies. Caution is necessary when using GPCP and CMAP rainfall to validate (1) the time-mean rainfall, as there are systematic differences over ocean and land between these two data sets, and (2) the timing of monsoon withdrawal over India, where the smooth southward progression seen in India Meteorological Department data is better realized in CMAP data compared to GPCP data. [ABSTRACT FROM AUTHOR]

Data analysis based on station observations reveals that many meteorological variables averaged over the Tibetan Plateau (TP) are closely correlated, and their trends during the past decades are well correlated with the rainfall trend of the Asian summer monsoon. However, such correlation does not necessarily imply causality. Further diagnosis confirms the existence of a weakening trend in TP thermal forcing, characterized by weakened surface sensible heat flux in spring and summer during the past decades. This weakening trend is associated with decreasing summer precipitation over northern South Asia and North China and increasing precipitation over northwestern China, South China, and Korea. An atmospheric general circulation model, the HadAM3, is employed to elucidate the causality between the weakening TP forcing and the change in the Asian summer monsoon rainfall. Results demonstrate that a weakening in surface sensible heating over the TP results in reduced summer precipitation in the plateau region and a reduction in the associated latent heat release in summer. These changes in turn result in the weakening of the near-surface cyclonic circulation surrounding the plateau and the subtropical anticyclone over the subtropical western North Pacific, similar to the results obtained from the idealized TP experiment in Part I of this study. The southerly that normally dominates East Asia, ranging from the South China Sea to North China, weakens, resulting in a weaker equilibrated Sverdrup balance between positive vorticity generation and latent heat release. Consequently, the convergence of water vapor transport is confined to South China, forming a unique anomaly pattern in monsoon rainfall, the so-called 'south wet and north dry.' Because the weakening trend in TP thermal forcing is associated with global warming, the present results provide an effective means for assessing projections of regional climate over Asia in the context of global warming. [ABSTRACT FROM AUTHOR]

Numerical experiments with different idealized land and mountain distributions are carried out to study the formation of the Asian monsoon and related coupling processes. Results demonstrate that when there is only extratropical continent located between 0 and 120°E and between 20/30°N and the North Pole, a rather weak monsoon rainband appears along the southern border of the continent, coexisting with an intense intertropical convergence zone (ITCZ). The continuous ITCZ surrounds the whole globe, prohibits the development of near-surface cross-equatorial flow, and collects water vapor from tropical oceans, resulting in very weak monsoon rainfall. When tropical lands are integrated, the ITCZ over the longitude domain where the extratropical continent exists disappears as a consequence of the development of a strong surface cross-equatorial flow from the winter hemisphere to the summer hemisphere. In addition, an intense interaction between the two hemispheres develops, tropical water vapor is transported to the subtropics by the enhanced poleward flow, and a prototype of the Asian monsoon appears. The Tibetan Plateau acts to enhance the coupling between the lower and upper tropospheric circulations and between the subtropical and tropical monsoon circulations, resulting in an intensification of the East Asian summer monsoon and a weakening of the South Asian summer monsoon. Linking the Iranian Plateau to the Tibetan Plateau substantially reduces the precipitation over Africa and increases the precipitation over the Arabian Sea and the northern Indian subcontinent, effectively contributing to the development of the South Asian summer monsoon. [ABSTRACT FROM AUTHOR]

The regional monsoons of the world have long been viewed as seasonal atmospheric circulation reversal-analogous to a thermally-driven land-sea breeze on a continental scale. This conventional view of monsoons is now being integrated at a global scale and accordingly, a new paradigm has emerged which considers regional monsoons to be manifestations of global-scale seasonal changes in response to overturning of atmospheric circulation in the tropics and subtropics, and henceforth, interactive components of a singular Global Monsoon (GM) system. The paleoclimate community, however, tends to view 'paleomonsoon' (PM), largely in terms of regional circulation phenomena. In the past decade, many high-quality speleothem oxygen isotope (δO) records have been established from the Asian Monsoon and the South American Monsoon regions that primarily reflect changes in the integrated intensities of monsoons on orbital-to-decadal timescales. With the emergence of these high-resolution and absolute-dated records from both sides of the Equator, it is now possible to test a concept of the 'Global-Paleo-Monsoon' (GPM) on a wide-range of timescales. Here we present a comprehensive synthesis of globally-distributed speleothem δO records and highlight three aspects of the GPM that are comparable to the modern GM: (1) the GPM intensity swings on different timescales; (2) their global extent; and (3) an anti-phased inter-hemispheric relationship between the Asian and South American monsoon systems on a wide range of timescales. [ABSTRACT FROM AUTHOR]

The effects of extratropical dynamics on the interannual variations in South-Asian Monsoon (SAM) are examined. Based on NCEP/NCAR reanalysis and CRU precipitation data, a conditional maximum covariance analysis is performed on sea level pressure, 200 hPa geopotential heights and the SAM rainfall by removing the linear effects of El-Niño Southern Oscillation from the fields. It is found that two modes provide a strong connection between the upper-level circulation in the Atlantic/European region and SAM rainfall: the Circumglobal Teleconnection (CGT) and the Summer North Atlantic Oscillation (SNAO). The structures in the 200 hPa heights of both modes in the Atlantic region are similar in the Atlantic region, and their southeastward extension to South Asia (SA) also corresponds to upper-level ridges (in their positive phases) in slightly different positions. Nevertheless, the influence of both modes on SAM rainfall is distinct. Whereas a positive CGT is related to a widespread increase of rainfall in SAM, a positive SNAO is related to a precipitation dipole with its positive phase over Pakistan and the negative phase over northern India. The physical mechanisms for the influence of CGT and SNAO on SAM are related to the upper-level geopotential anomaly which affects the amplitude and position of the low-level convergence. The small displacements of the centers of these responses and the low level cold advection from the north east of SA in case of SNAO explain the differences in the corresponding SAM rainfall distributions. These findings are confirmed with the relatively high-resolution coupled climate model EC-Earth, which gives confidence in the physical basis and robustness of these extratropical variability modes and their influence on the South-Asian monsoon rainfall. [ABSTRACT FROM AUTHOR]

Northern Tibetan Plateau uplift and global climate change are regarded as two important factors responsible for a remarkable increase in dust concentration originating from inner Asian deserts during the Pliocene-Pleistocene period. Dust cycles during the mid-Pliocene, last glacial maximum (LGM), and present day are simulated with a global climate model, based on reconstructed dust source scenarios, to evaluate the relative contributions of the two factors to the increment of dust sedimentation fluxes. In the focused downwind regions of the Chinese Loess Plateau/North Pacific, the model generally produces a light eolian dust mass accumulation rate (MAR) of 7.1/0.28 g/cm/kyr during the mid-Pliocene, a heavier MAR of 11.6/0.87 g/cm/kyr at present, and the heaviest MAR of 24.5/1.15 g/cm/kyr during the LGM. Our results are in good agreement with marine and terrestrial observations. These MAR increases can be attributed to both regional tectonic uplift and global climate change. Comparatively, the climatic factors, including the ice sheet and sea surface temperature changes, have modulated the regional surface wind field and controlled the intensity of sedimentation flux over the Loess Plateau. The impact of the Tibetan Plateau uplift, which increased the areas of inland deserts, is more important over the North Pacific. The dust MAR has been widely used in previous studies as an indicator of inland Asian aridity; however, based on the present results, the interpretation needs to be considered with greater caution that the MAR is actually not only controlled by the source areas but the surface wind velocity. [ABSTRACT FROM AUTHOR]

Using large-scale variables, in this study we have developed a method for defining monsoon onset/retreat in the Australia-Asian region and used this method to study monsoon activities simulated by global climate models. For this purpose, the method needs to capture fundamental characteristics of monsoon rainfall and circulation seasonal variations and at the same time it can be reasonably simulated by current climate models. We develop the method by using both atmospheric precipitable water and wind conditions in our definition and compared our results using 44-year ERA-40 reanalysis data with some published results in the region. Our results offer similar features to several observational studies, including features in Australia-Asian summer monsoon temporal and spatial evolutions and their interannual variations. Results further show that the observed significant increase in summer rainfall in northwest Australia corresponds to earlier onset and much longer duration of its summer monsoon, with its duration significantly increased. Prolonged summer monsoon duration is also seen in central-east China where upward rainfall trend is observed. Furthermore, the Australian summer monsoon appears to be more affected by ENSO than the Asian monsoon, with delayed onsets and shortened durations during El Nino years. Finally, by analyzing results from an IPCC AR4 model, we have shown that using the two large-scale variables simulated by climate models, it is possible to conduct some detailed studies on monsoon activities in current and future climate. Results from this particular model suggest that global warming could potentially modify some of the monsoon characteristics, including earlier onset in most of the region but different features for changes in duration. In the Australian region, it also displays further southward penetration of its summer monsoon. [ABSTRACT FROM AUTHOR]